Spondylosyndesis, or spinal fusion, is a surgical technique used to combine two or more vertebrae into a single, rigid working unit. This is typically achieved by introducing a supplementary bone tissue, such as an autograft or allograft, into the intervertebral space between two target vertebrae, at the location that is typically occupied by an intervertebral disc. The supplementary bone tissue is then used in conjunction with the patient's natural osteoblastic processes in order to grow bone or osseous tissue between the two or more target vertebrae, which acts to fuse them together into the desired rigid unit. This procedure is used primarily to eliminate pain that is caused by abnormal motion of one or both of the target vertebrae; pain relief occurs by immobilizing the vertebrae themselves and preventing the abnormal motion. Alternatively, surgically implantable synthetic intervertebral fusion cages or devices may be used to perform spinal fusion procedures.
Surgically implantable intervertebral fusion cages are well known in the art and have been actively used to perform spinal fusion procedures for many years. Their use became popularized during the mid 1990's with the introduction of the BAK Device from the Zimmer Inc. The BAK system is a fenestrated, threaded, cylindrical, titanium alloy device that is capable of being implanted into a patient as described above through an anterior or posterior approach, and is indicated for cervical and lumbar spinal surgery. Most common spinal fusion systems today are made from metals, such as titanium or cobalt chrome alloys, or from a polymer such as polyetheretherketone (PEEK) which is commonly used in biomedical implants. Unfortunately, these implant materials have a modulus which is much higher than that of bone and there is clinical evidence of implant subsidence and movement which is believed to be attributable to mechanical incompatibility between natural bone and the implant material. Also bone pressure necrosis does occur as a result of the presence of these metal implants.
Implants based on bone material from a donor (allograft) or from the patient itself (autograft) do have an inconsistent mechanical strength and show subsidence over time. The inconsistent properties of these implants make them generally unpredictable, challenging to reliably machine and especially prone to migration and expulsion due to the difficulty of consistently machining teeth into the upper and lower implant contact surfaces.
Although titanium alloy cages give good fusion rates, their modulus is significantly dissimilar to human bone. The stress transfer between an implant device and a bone is not homogeneous when Young's moduli of the implant device and the bone are different. This results in stress shielding. In such conditions, bone atrophy occurs and leads to the loosening of at the implant bone interface and eventually lead to failure. Therefore, the stiffness (Young's modulus) of the implant is preferably not too high compared to that of bone. Implant devices made from metallic biomaterials such as stainless steels, Co—Cr alloys, and titanium (Ti) and its alloys have a Young's modus generally much greater than that of the bone. Young's moduli of the most widely used stainless steel for implant devices, SUS316L stainless steel and Co—Cr alloys, are around 180 GPa and 210 GPa, respectively. Young's moduli of Ti (pure titanium) and its alloys are generally smaller than those of stainless steels and Co—Cr alloys. For example, Ti and its alloy, Ti-6Al-4V ELI, which are widely used for constructing implant devices, have a Young's modulus of around 110 GPa. However, this value is still higher than that of the bone, which is on the order of 10-30 GPa.
The foregoing shortcomings in the spinal fusion cage arts apply to other orthopedic implants as well, such as intermedullary nails for long bones such as the femur.
Therefore, there remains a need for a biostable implant such as for use as an orthopedic implant or plate which has a tensile modulus comparable to that of bone, which does not subside and provides a good stability.